1. Field of the Invention
The present invention generally relates to acoustic wave devices, and more particularly, to an acoustic wave device having a seal portion having a cavity located above an acoustic wave element.
2. Description of the Related Art
A surface acoustic wave device (SAW device) is capable of exciting an acoustic wave by applying power to comb electrodes of an interdigital transducer (IDT) formed on a piezoelectric substrate. The SAW device is widely used for various circuits that process radio signals in a frequency band of for example, 45 MHz to 2 GHz. Examples of these circuits are bandpass filters for transmission, bandpass filters for reception, local oscillation filters, antenna duplexers, intermediate frequency filters, and FM modulators.
Recently, an acoustic wave device using a film bulk acoustic resonator (FBAR) has been developed. The FBAR has a pair of electrodes provided on opposite surfaces of a piezoelectric thin-film and utilizes a vibration in the thickness direction. The acoustic wave device using the FBAR particularly has good performance at higher frequencies and is frequently used in the frequency range of 1 GHz to 10 GHz.
The recent progress in the field of mobile communications is remarkable and needs downsizing of signal processing equipment, which may be realized by miniaturizing electronic components such as acoustic wave devices. In order to realize desired performance of the acoustic wave device, a void space is provided above a functional portion of an acoustic wave element. The functional portion of the acoustic wave element formed by a surface acoustic wave element is the interleaving electrode fingers of the IDT. The functional portion of the acoustic wave element formed by an FBAR is a region in which the upper and lower electrodes face each other across the piezoelectric thin-film.
There are some proposals of using a seal portion having a cavity located above the functional portion of the acoustic wave element in order to meet the above requirements. Japanese Patent No. 3291046 (hereinafter, simply referred to as document D1) proposes to use a soluble resin formed in a region that is to become a cavity above the acoustic wave element. A plate is provided on the soluble resin and is then removed, so that a cavity structure can be realized. Japanese Patent Application Publication No. 2003-523082 (hereinafter, simply referred to as document D2) proposes to form a frame structure that surrounds electric components. An auxiliary film is attached on the frame structure so as to form a cavity above the electric components, and a resin layer is then formed on the auxiliary film. Thereafter, the layers except a roof portion of the frame structure are removed, so that a cavity structure can be realized. Japanese Patent No. 3196693 (hereinafter, simply referred to as document D3) proposes to attach a resin film on a piezoelectric substrate on which acoustic wave elements are formed. Next, an opening is formed in a part of the resin film located above the functional portion of the substrate on which the acoustic wave elements are formed. Then, a circuit board is adhered on the resin film, so that a cavity structure can be realized. Japanese Patent No, 3225906 (hereinafter, simply referred to as document D4) proposes to use a photosensitive resin on a substrate on which acoustic wave elements are provided. Next, openings are formed in the photosensitive resin at positions located above the functional portions of the acoustic wave elements. Then, a substrate formed by a group of wiring boards is mounted on the photosensitive resin. After that, the whole structure is divided into parts by dicing, so that cavity structures can be realized.
However, the acoustic devices formed by the arts disclosed in documents D1 through D4 have a disadvantage that the devices does not withstand pressure applied at the time of modularizing and a ceiling portion of the cavity structure is concaved. The above disadvantage may be removed by thickening the ceiling portion of the cavity structure. However, an increased amount of resin for the purpose of obtaining an increased thickness of the ceiling portion of the cavity structure may degrade the yield.